1. Field of the Invention
The present invention relates to a fluid pressure booster which generates assist force corresponding to an operating amount of a brake operation member by using fluid pressure supplied from an auxiliary fluid pressure source and applies assisted force to a master cylinder, and a fluid pressure brake apparatus having the fluid pressure booster.
2. Description of Related Art
For example, U.S. Pat. No. 4,548,037 B discloses a basic technique of a fluid pressure booster which adjusts fluid pressure supplied from an auxiliary fluid pressure source having a power-operated pump and a pressure accumulator, to a value corresponding to an operating amount of a brake operation member with a pressure adjusting device having a spool valve, introduces the same into a boost chamber, applies the fluid pressure to a boost piston to generate assist force corresponding to the brake operating amount and applies assisted force (force obtained by adding the assist force to brake operating force applied by a vehicle driver) to a piston of a master cylinder.
Also, there has been known a fluid pressure brake apparatus having a reflux-type pressure adjusting unit which performs an ABS (antilock brake system) control or ESC (Electronic Stability Control) operation based on a command from an electronic control unit.
When the electronic control unit determines that it is necessary to decrease pressure of a wheel cylinder, based on information from a variety of sensors detecting wheel speed, operating stroke of a brake operation member, brake fluid pressure, movement of a vehicle and the like, the reflux-type pressure adjusting unit cuts off a fluid pressure path extending from the master cylinder to the wheel cylinder by a pressure increase solenoid valve and connects the wheel cylinder to a low pressure fluid storage by a pressure decrease solenoid valve, thereby performing pressure decrease control.
After that, when the electronic control unit determines that it is necessary to perform the pressurization again, the reflux-type pressure adjusting unit drives a power-operated reflex pump to pump up brake fluid in the low pressure fluid storage, opens the pressure increase solenoid valve and closes the pressure decrease solenoid valve, thereby refluxing the pumped brake fluid to the fluid pressure path extending from the master cylinder to the wheel cylinder.
Regarding the fluid pressure brake apparatus adopting the reflux-type pressure adjusting unit, two types are suggested. In one type device, a cutoff valve is provided at a more upstream side (master cylinder-side) than a position (reflux point) at which the brake fluid pumped by the reflux pump is introduced to the fluid pressure path extending from the master cylinder to the wheel cylinder, and the cutoff valve is closed when the control such as ABS is performed. In the other type device, the cutoff valve is not provided.
In the latter device having no cutoff valve, the brake fluid pumped by the reflux pump flows backward toward the master cylinder (hereinafter referred to as ‘pump back’).
According to the related-art fluid pressure brake apparatus having the reflux-type pressure adjusting unit, regarding a booster which is provided so as to assist a braking operation of a driver, a vacuum booster has been adopted which generates assist force by using a negative pressure of an engine. However, since it is not possible to expect the assist by the negative force of the engine in a valve matic vehicle in which an intake valve lift amount is continuously varied to enable an intake value to function as a throttle valve, an HEV (hybrid electric vehicle), an EV (Electric Vehicle) and the like. Therefore, it is considered to combine a fluid pressure booster. The fluid pressure booster applies fluid pressure (boost pressure) introduced into a boost chamber to a boost piston, thereby generating the assist force.
However, when the fluid pressure booster is adopted instead of the vacuum booster, the precision of the pressure adjustment is lowered due to the pump back, so that an operation feeling of the brake and the durability of a cup of the master cylinder are thus deteriorated. Therefore, measures capable of avoiding the problems are needed.
That is, when the pump back occurs, the piston of the master cylinder is pushed back by the fluid pressure flowing backward from the pressure adjusting unit. The pushing force is transferred to the boost piston (which is referred to as a power piston in the vacuum booster), so that the boost piston is also pushed back.
In the fluid pressure brake apparatus adopting the vacuum booster which operates a power piston by a pressure difference between a negative pressure chamber and an atmosphere chamber and thus generates the assist force, even when the power piston is pushed back, since the air sealed in the atmosphere chamber is compressed by displacement of the power piston, the pressure in the atmosphere chamber is not increased much and the influence of the pump back is suppressed to a slight amount.
Compared to the vacuum booster, according to the fluid pressure booster, brake fluid sealed in the boost chamber is compressed as the boost piston is pushed back due to the pump back, and the brake fluid is non-compressible oil. Therefore, the pressure increase in the boost chamber is not negligible until a discharge port, which enables the boost chamber to communicate with a reservoir, is opened.
For the fluid pressure brake apparatus in which electronic control of the wheel cylinder pressure is made by so-called differential pressure control, a pressure increase, which corresponds to the increase of the master cylinder pressure due to the pump back, occurs in the boost chamber, so that the master cylinder pressure is increased to balance with the boost pressure. Therefore, there would be caused the lowering of the control precision of the wheel cylinder pressure, which deteriorates the reliability of the ABS control and the like, the pulsation, which deteriorates the durability of the cup of the master cylinder and the like, the deterioration of the operation feeling of the brake and the like.
Also, in the fluid pressure brake apparatus in which the pressure adjusting control of the wheel cylinder by the electronic control is made by pulse control, when the boost pressure is increased by the pump back in a pressure boost mode, an introduction amount of the fluid pressure into the wheel cylinder is more increased than a target amount. Accordingly, there would be also caused the lowering of the control reliability, the deterioration of the operation feeling and the like although the degrees thereof are less than those in the fluid pressure brake apparatus adopting the differential pressure control.